Erosion shield for a wind turbine blade

ABSTRACT

An erosion shield for a wind turbine blade is described, the erosion shield having a plurality of layers of erosion resistant material. The layers of erosion resistant material have an adhesive bond strength between adjacent layers less than the cohesive tensile strength of the layers, such that the outer layers of erosion resistant material are arranged to peel away or delaminate from the erosion shield under the action of the wind once the particular layer is ruptured or eroded. This dynamic removal of the outer layers of the erosion shield provides for increased shield lifetime, and a reduction in the maintenance operations required for a wind turbine blade having such an erosion shield.

FIELD OF THE INVENTION

The present invention relates to an erosion shield for a wind turbineblade, and a wind turbine blade having such an erosion shield.

BACKGROUND OF THE INVENTION

During the lifetime of a wind turbine, considerable resources areexpended on continued maintenance operations to ensure optimum turbineperformance. With regard to the blades of a wind turbine, erosion at theleading edge of the blades is one area of attention.

It is known to provide an erosion shield at the leading edge of a windturbine blade. The erosion shield comprises a layer or coating ofresilient erosion resistant material which is applied along the lengthof the blade covering the leading edge. The erosion shield providesimproved resistance to erosion, being usually formed of a resilientmaterial as opposed to the relatively brittle fibre composite materialused to produce the body of a wind turbine blade, and accordingly actsto improve the overall durability of the wind turbine blade. An exampleof an erosion shield comprising a polymeric film can be seen in EP 2 153065.

Field experience has shown that such erosion shields or erosion tapeswill last approximately between 5-8 years, depending on theenvironmental conditions of the location of the wind turbine, as well asthe turbine operating conditions, especially blade tip speed. However,when the erosion resistant layer ruptures due to heavy erosion, ingeneral there will be portions of the layer left on the turbine blade tofreely flutter in the wind. This fluttering will result in a loss ofaerodynamic performance of the blade, as well as the generation ofadditional aerodynamic noise.

In the case of such a rupture of the erosion shield, such reducedturbine performance will continue until a maintenance operation isperformed to remove the ruptured shield. This involves an extensive andcomplicated operation to stop operation of the turbine, remove theruptured shield from along the leading edge of the turbine blade, and toapply a new erosion shield along the leading edge.

It is an object of the invention to provide an erosion shield for a windturbine blade which reduces the problems associated with shield rupture,and requires reduced maintenance.

SUMMARY OF THE INVENTION

Accordingly, there is provided an erosion shield for a wind turbineblade, the erosion shield having an external surface exposed at aleading edge of the wind turbine blade and comprising:

-   -   a plurality of layers of erosion resistant material, said        plurality of layers arranged in a stack from an outermost layer        substantially forming said external surface to an innermost        layer arranged to be attached to a leading edge of a wind        turbine blade, said plurality of layers bonded to adjacent        layers in the stack,    -   wherein said plurality of layers of erosion resistant material        have a cohesive strength or tensile strength greater than the        adhesive strength or bond strength between adjacent or        subsequent layers, such that at least a section of an outermost        exposed layer will delaminate or peel from said erosion shield        under the action of wind when at least a portion of said section        of said outermost exposed layer has been eroded or ruptured, to        present a relatively smooth external surface of said erosion        shield.

As the outermost layer of erosion resistant material is gradually wornaway or eroded during use of the blade, portions of the layer may detachfrom the surface of the erosion shield and flutter or flap in the wind.Due to the relative weakness of the bond between adjacent layers of theerosion resistant material, once such a break or rupture occurs in theoutermost layer, the force of the wind overcomes the relatively weakbonding between successive layers, and acts to delaminate or peel offthe remaining section of the layer to remove any free flutteringportions of the erosion resistant material from the erosion shield.

This dynamic removal of outer layers of the erosion shield once suchlayers fail reduces aerodynamic noise produced by such failures, as wellas eliminates any negative aerodynamic performance as a result offluttering or flapping sections of the erosion shield. It also providesfor reduced maintenance of wind turbine blades in service, as the timebetween service operations to replace or clean an erosion shield of awind turbine blade is increased.

It will be understood that the erosion shield may be arranged to detachor delaminate an entire layer of erosion resistant material at once, orto delaminate individual sections of the exposed layer or layers whichcontain the location where the rupture or failure of the erosion shieldoccurs.

Preferably, said plurality of layers are adhesively bonded to adjacentlayers in the stack, and wherein said plurality of layers of erosionresistant material have a cohesive strength greater than the adhesivebond strength between adjacent layers.

While the normal requirement for a good adhesion is above 5 MPa inpull-off according to ISO 4624 with cohesive failure mode, preferablythe adhesive bond strength, in particular the pull-off adhesivestrength, between adjacent layers is below 5 MPa, preferably below 2MPa, with adhesive failure mode between the adjacent layers. Such arelatively low adhesive bond strength ensures that adjacent layers willeasily delaminate when an outer layer has been eroded or ruptured, toensure dynamic removal of layers of the erosion shield.

Preferably, said layers of erosion resistant material comprise an innersurface facing away from the external surface of the erosion shield andan outer surface facing towards the external surface of the erosionshield,

-   -   wherein said erosion shield further comprises at least one layer        of bridging material located between adjacent layers of erosion        resistant material to bond adjacent inner and outer surfaces of        adjacent layers of erosion resistant material in said stack.

The erosion shield is formed as an interleaved structure of erosionresistant material and bridging material.

In one embodiment, said at least one bridging layer comprises a layer ofadhesive. The adhesive layers are provided as a disposable adhesivelayer between layers of the erosion resistant material, which will bediscarded when the outer layer of erosion resistant material delaminatesor peels away from the erosion shield. The bridging layer may comprise athermoplastic material, and/or a coating material that will work as asemi-resistant erosion layer as well as functioning as an adhesivebetween adjacent layers of the erosion resistant material, as thecoating material cures after being applied.

Preferably, said at least one bridging layer has a tensile cohesivestrength less than the tensile cohesive strength of said layers oferosion resistant material.

Preferably, the elongation-to-break for the bridging layer is lower thanthe elongation-at-break of the erosion resistant material.

The erosion shield may be formed as a single member having interleavedlayers of different substances having different material properties,e.g. the erosion resistant material may comprise a plastic, malleablematerial which will substantially absorb erosion forces over time, whilethe bridging layer may comprise a flexible material which is operable toeasily peel away from the surface of the shield under the action ofwind.

Additionally or alternatively, the bridging layer may comprise a brittleor frangible material which can easily break away from the shield, toremove ruptured portions of the erosion resistant material. In such acase, the tensile strength of the bonding layer is chosen to be easilybreakable or frangible, such that the bridging layer can be easilybroken off from the erosion shield.

The manufacture of such an erosion shield may be done throughco-extrusion of two thermoplastics having different material properties.

Preferably, said at least one layer of bridging material has arelatively stronger bond to an adjacent inner surface of a layer oferosion resistant material than to an adjacent outer surface of a layerof erosion resistant material.

The bridging material is better bonded to the outer layer of the erosionshield, such that the bridging material is discarded or peeled away withthe eroded outer layer. For such a construction, the bridging layer maybe formed integrally with the outer adjacent layer of erosion resistantmaterial, and adhesively bonded to the outer face of the inner adjacenterosion material layer.

In one embodiment, the bridging layer may be formed by an adhesive tapeapplied to the underside or inner surface of each layer of erosionmaterial, to bond to the succeeding erosion material layer. In thiscase, the tape may be applied to the inner surface of the precedinglayer of erosion resistant material using a strong adhesive, while thebond to the outer surface of the succeeding layer of erosion resistantmaterial is made using a low surface tension adhesive.

For example, pressure sensitive acrylic adhesives provide a relativelyhigher bond strength compared to a pressure sensitive silicone basedadhesive. In this regard, the bridging layer may be bonded to the innersurface of the preceding layer of erosion resistant material using apressure sensitive acrylic adhesive, and bonded to the outer surface ofthe succeeding layer of erosion resistant material is made using apressure sensitive silicone based adhesive.

Preferably, said erosion shield is arranged to extend in a lengthwisedirection along a portion of a leading edge of a wind turbine blade, andwherein said layers of erosion resistant material are divided into aplurality of sections along the length of the erosion shield, whereinsaid sections of said erosion resistant layers are arranged toindividually delaminate when at least a portion of an erosion resistantlayer within a particular section has been eroded or ruptured.

The layers of the erosion shield may be divided into different sectionsalong the length of the leading edge, such that individual sections areseparately or independently peelable. Accordingly, if the erosion shieldlayer only ruptures in one part of the erosion shield, then the layerwill only be peeled away in the vicinity of that particular rupturepoint. This prevents the disposal of an entire layer of the erosionshield if there has been only a single minor rupture of the layer,thereby prolonging the lifetime of the entire erosion shield.

Preferably, said plurality of sections are provided towards the tip endof the wind turbine blade.

Due to the higher speed at the tip end of the blade, the erosion isnormally seen on more occasions at the blade tip end. Accordingly,sections of an erosion shield arranged at a tip end of a blade will ingeneral undergo more erosion than sections towards the root end of theblade.

Preferably, the erosion shield has a tip end for location towards thetip end of a wind turbine blade and a root end for location towards theroot end of a wind turbine blade, said plurality of sections having asection width measured in a longitudinal direction between the root endand the tip end of said erosion shield, wherein said plurality ofsections are arranged such that the section width of individual sectionsdecreases moving from the root end to the tip end of the erosion shield.

In one embodiment, the erosion shield has a tip end for location towardsthe tip end of a wind turbine blade and a root end for location towardsthe root end of a wind turbine blade, wherein the number of layers orerosion resistant material provided at the tip end of the erosion shieldis greater than the number of layers of erosion resistant materialprovided at the root end of the erosion shield.

As the tip end of the erosion shield will experience greater erosionthan the root end, accordingly the provision of a greater number oflayers of erosion resistant material at the tip end will lead to a moreeffective operation of the erosion shield, and a longer lifetime betweenreplacement operations.

Preferably, a boundary between adjacent sections of said layers oferosion resistant material is defined by a weakened or perforated lineor strip in at least one of said layers of erosion resistant material orsaid layers of bridging material, preferably in said layers of bridgingmaterial.

A perforated line provides for a relatively simple and well-defined tearpoint between adjacent sections of said layers. Additionally oralternatively, a boundary between adjacent sections of said layers oferosion resistant material may be defined by a frangible or breakableportion between adjacent sections. Additionally or alternatively, aboundary between adjacent sections of said layers may be defined by ascored or weakened section of said layers.

In a further embodiment, the erosion shield may comprise a perforated,weakened, scored or frangible section arranged in a lengthwise directionparallel to the leading edge of the erosion shield, to allow for easierdelamination or peeling of layers of the erosion shield away from saidleading edge. In such a case, preferably such a breakable section isdefined in the bridging layers between adjacent layers of erosionresistant material. Such a lengthwise breakable section of the erosionshield may be located on the erosion shield at the boundary pointbetween the pressure side and the suction side of a wind turbine toreceive the erosion shield, or may be positioned at an alternativelocation, e.g. at a location on the erosion shield which corresponds tothe preferred angle of attack of oncoming wind at the wind turbineblade.

In one embodiment, said layers of erosion resistant material are formedfrom a first resilient material and a second flexible material, saidfirst resilient material provided on said erosion shield to cover aleading edge of a wind turbine blade, said second flexible materialarranged to extend from said first resilient material at either side ofsaid leading edge.

Providing the erosion resistant layers as a composite of a resilientmaterial at the leading edge and a flexible material depending from theleading edge results in a high degree of erosion resistance, due to theresilient material, but which is relatively easily peelable from theerosion shield once said resilient material has been eroded or ruptured.

In an additional or alternative embodiment, said plurality of layers oferosion resistant material are bonded to adjacent layers in the stackusing at least one bonding area, wherein said at least one bonding areabetween adjacent layers of erosion resistant material is arrangedadjacent the leading edge of the erosion shield.

In this embodiment, preferably the layers of erosion resistant materialare not bonded to each other at the leading edge. This allows the layersto more easily peel from the erosion shield, as once a layer is rupturedit will peel back from the point of rupture to the bonding area.Accordingly, the unbounded section of the layer will present a greatersurface area to the oncoming wind, and the delamination force exerted onthe layer will be greater.

Preferably, a cavity or channel is defined in said layers of bridgingmaterial, said channel or cavity arranged adjacent the underside ofinner surface of the outer adjacent layer of erosion resistant material,preferably at the leading edge of the erosion shield.

The use of a cavity adjacent the undersurface of a layer of erosionresistant material means that the erosion resistant layer is not bondedto the underlying layer at the leading edge. Thus, once a rupture orerosion of the outer layer occurs at the leading edge, the layer oferosion resistant material will more easily delaminate from the erosionshield, improving shield performance.

Preferably, said channel is a through-going aperture extending throughsaid layer of bridging material.

Preferably, said layers of erosion resistant material are approximately0.1-0.5 mm deep, preferably 0.3 mm deep.

Preferably, said at least one layer of bridging material isapproximately 0.05-0.25 mm deep, preferably 0.1 mm deep.

In one embodiment, the plurality of layers of erosion-resistant materialmay be formed from different types of erosion resistant materials, saidmaterials having various different properties.

Preferably, said erosion resistant material may be selected from thefollowing available materials: W8607 from 3M, or 54994 PV3 from tesa SE.

Preferably, said bridging material may be selected from the followingavailable materials: STEODUR-PUR-Kantenschutz from Bergolin; ALEXITLeading Edge Protection 442-52 from Mankiewicz; XA258 from Hempel; orOldodur Blade Finish from Relius.

There is also provided a wind turbine blade having an erosion shield asdescribed above.

In particular, the wind turbine blade extends in a longitudinaldirection parallel to a longitudinal axis and comprises a tip end and aroot end, the wind turbine blade comprising a profiled contour includinga pressure side and a suction side, as well as a leading edge and atrailing edge with a chord having a chord length extending therebetween,the profiled contour, when being impacted by an incident airflow,generating a lift, wherein the erosion shield is provided along at leasta section of said leading edge.

In one embodiment, the wind turbine blade comprises a channel formed atthe leading edge of the blade, said channel for receiving said erosionshield such that the erosion shield is provided flush with the surfacesof the blade adjacent the channel and the erosion shield.

There is further provided a wind turbine having at least one windturbine blade comprising an erosion shield as described above.

In particular, the wind turbine comprises a wind turbine blade for arotor of a wind turbine having a substantially horizontal rotor shaft,the rotor comprising a hub, from which the wind turbine blade extendssubstantially in a radial direction when mounted to the hub.

DESCRIPTION OF THE INVENTION

An embodiment of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 shows a wind turbine;

FIG. 2 shows a schematic view of a wind turbine blade according to theinvention;

FIG. 3 shows a schematic view of an airfoil profile of the blade of FIG.2;

FIG. 4 shows a cross-sectional view of an erosion shield for a windturbine blade according to the invention;

FIGS. 5(a)-(c) illustrate in partial cross-section the operation of theerosion shield of FIG. 4 in use on the blade of FIG. 2;

FIG. 6 shows a perspective view of a portion of an embodiment of erosionshield according to the invention;

FIG. 7 shows a cross-sectional view of a further embodiment of erosionshield according to the invention; and

FIG. 8 shows a cross-sectional view of a further embodiment of erosionshield according to the invention.

Common elements between the different embodiments will be referred tousing the same reference numerals.

FIG. 1 illustrates a conventional modern upwind wind turbine accordingto the so-called “Danish concept” with a tower 4, a nacelle 6 and arotor with a substantially horizontal rotor shaft. The rotor includes ahub 8 and three blades 10 extending radially from the hub 8, each havinga blade root 16 nearest the hub and a blade tip 14 furthest from the hub8. The rotor has a radius denoted R.

FIG. 2 shows a schematic view of a first embodiment of a wind turbineblade 10 according to an embodiment of the invention. The wind turbineblade 10 has the shape of a conventional wind turbine blade andcomprises a root region 30 closest to the hub, a profiled or an airfoilregion 34 furthest away from the hub and a transition region 32 betweenthe root region 30 and the airfoil region 34. The blade 10 comprises aleading edge 18 facing the direction of rotation of the blade 10, whenthe blade is mounted on the hub, and a trailing edge 20 facing theopposite direction of the leading edge 18.

The airfoil region 34 (also called the profiled region) has an ideal oralmost ideal blade shape with respect to generating lift, whereas theroot region 30 due to structural considerations has a substantiallycircular or elliptical cross-section, which for instance makes it easierand safer to mount the blade 10 to the hub. The diameter (or the chord)of the root region 30 is typically constant along the entire root area30. The transition region 32 has a transitional profile 42 graduallychanging from the circular or elliptical shape 40 of the root region 30to the airfoil profile 50 of the airfoil region 34. The chord length ofthe transition region 32 typically increases substantially linearly withincreasing distance r from the hub.

The airfoil region 34 has an airfoil profile 50 with a chord extendingbetween the leading edge 18 and the trailing edge 20 of the blade 10.The width of the chord decreases with increasing distance r from thehub.

It should be noted that the chords of different sections of the bladenormally do not lie in a common plane, since the blade may be twistedand/or curved (i.e. pre-bent), thus providing the chord plane with acorrespondingly twisted and/or curved course, this being most often thecase in order to compensate for the local velocity of the blade beingdependent on the radius from the hub.

FIG. 3 shows a schematic view of an airfoil profile 50 of a typicalblade of a wind turbine depicted with the various parameters, which aretypically used to define the geometrical shape of an airfoil. Theairfoil profile 50 has a pressure side 52 and a suction side 54, whichduring use—i.e. during rotation of the rotor—normally face towards thewindward (or upwind) side and the leeward (or downwind) side,respectively. The airfoil 50 has a chord 60 with a chord length cextending between a leading edge 56 and a trailing edge 58 of the blade.The airfoil 50 has a thickness t, which is defined as the distancebetween the pressure side 52 and the suction side 54. The thickness t ofthe airfoil varies along the chord 60. The deviation from a symmetricalprofile is given by a camber line 62, which is a median line through theairfoil profile 50. The median line can be found by drawing inscribedcircles from the leading edge 56 to the trailing edge 58. The medianline follows the centres of these inscribed circles and the deviation ordistance from the chord 60 is called the camber f. The asymmetry canalso be defined by use of parameters called the upper camber and lowercamber, which are defined as the distances from the chord 60 and thesuction side 54 and pressure side 52, respectively.

Airfoil profiles are often characterised by the following parameters:the chord length c, the maximum camber f, the position df of the maximumcamber f, the maximum airfoil thickness t, which is the largest diameterof the inscribed circles along the median camber line 62, the positiondt of the maximum thickness t, and a nose radius (not shown). Theseparameters are typically defined as ratios to the chord length c.

With reference to FIG. 5, an erosion shield according to the inventionis indicated generally at 70. The erosion shield 70 comprises an outerleading edge surface 70 a, and an inner blade surface 70 b which isarranged to be applied to the leading edge 56 of a wind turbine blade10.

The erosion shield 70 comprises a plurality of layers of erosionresistant material 72, interleaved with layers of a bridging material74, to create a stacked structure. The outer leading edge surface 70 ais substantially formed by the outermost layer of erosion resistantmaterial 72. Preferably, the erosion shield 70 further comprises anadhesive layer 76 provided at said inner blade surface 70 b, saidadhesive layer 76 arranged to attach the erosion shield 70 to a bladeleading edge 56.

The erosion resistant material 72 is preferably formed by a resilientmaterial which has improved erosion resistant properties relative to therelatively fragile material which forms the body of a wind turbine blade10.

The bridging material 74 is formed from a substantially disposable orweakened material, such that there is a relatively low adhesive bondstrength between subsequent layers of the erosion resistant material 72.This allows for the erosion resistant material 72 to relatively easilypeel away from or delaminate from the erosion shield 70 in the event ofa layer rupture.

While only three layers of erosion resistant material 72 and bridgingmaterial 74 are shown in the embodiment of FIG. 4, it will be understoodthat the erosion shield 70 may comprise any number of suitable layers.This may be selected depending on the desired lifetime of the erosionshield 70, as a greater number of layers of erosion resistant material72 would result in a longer lifetime of the erosion shield 70 beforereplacement is required.

With reference to FIGS. 5(a), (b) and (c), the operation of an erosionshield 70 on a wind turbine blade according to the invention isillustrated. Only the outer layers of erosion resistant and bridgingmaterials 72,74 are shown.

FIG. 5(a) illustrates the erosion shield 70 under normal operation on awind turbine blade (not shown). The outermost exposed layer of theerosion resistant material 72 is provided at the leading edge of theblade, and is acted upon by the oncoming wind, indicated by arrows A.The erosion resistant material 72, being more resistant to erosion thanthe relatively brittle fibre composite material forming the body of thewind turbine blade, is better able to withstand the abrasive action ofthe wind, and any foreign objects, sand, salt, etc. carried by the windA.

After a period of time however, the outer layer of the erosion resistantmaterial 72 will rupture due to the continued erosive and abrasiveaction of the wind A, as shown in FIG. 5(b). At the location where theerosion resistant layer 72 ruptures, the layer of erosion resistantmaterial 72 will form edges or free ends 72 a,72 b. The wind A willgenerally act to flutter said free ends 72 a,72 b, by acting on the nowexposed underside of the outermost layer 72.

It will be understood that the free ends 72 a,72 b shown in FIG. 5(b)may be exaggerated for the purpose of clarity, and that an initialrupture of an outer layer of material may not result in freelyfluttering free ends of material, rather an aperture defined in theouter layer which, over continued erosive action, may result in freeends as shown in FIG. 5(b).

With reference to FIG. 5(c), the action of the wind A on the free ends72 a,72 b of the outermost layer of erosion resistant material 72 willact to force the erosion resistant layer 72 back from the point ofrupture at the leading edge of the erosion shield 70. Due to thepresence of the underlying weakened or disposable layer 74, theoutermost erosion resistant layer 72 is easily delaminated from theerosion shield without a substantial amount of force, e.g. the outermostlayer 72 can be dynamically delaminated from the erosion shield 70solely under action of the wind A.

As the outermost layer of erosion resistant material 72, as well as theunderlying layer of bridging material 74, is peeled away from theerosion shield 70 by the wind A, the next layer of erosion resistantmaterial of the shield 72 is exposed to the wind A. The erosion shield70 accordingly presents a relatively smooth erosion resistant surface,without any loss of aerodynamic efficiency or increase in aerodynamicnoise due to the continued presence of fluttering ends of layers of theerosion shield at the leading edge of the blade.

The above cycle of rupture and dynamic delamination or peeling isrepeated iteratively for each layer of erosion resistant material 72provided in the erosion shield 70. Accordingly, the erosion shield 70 isarranged to dynamically shed outer layers of the erosion resistantmaterial 72 once such layers have been used, without requiring extensivemaintenance operations.

The bridging layer material 74 is chosen such that the adhesive bondstrength between adjacent layers of the erosion resistant material 72 isless than the cohesive tensile strength of the erosion resistantmaterial 72. Accordingly, once the outer layer of erosion resistantmaterial has been eroded or ruptured, the peel strength required todelaminate the ruptured layer from the shield 70 is relatively low, andmay be accomplished simply under action of the wind.

A normal indicator of good adhesive bond strength is that the pull-offstrength under ISO 4624 is above 5 MPa with cohesive failure mode.Accordingly, the adhesive bond strength between adjacent layers of theerosion resistant material 72 of the erosion shield 70 according to theinvention is below 5 MPa, preferably under 2 MPa.

The erosion resistant material 72 may be any material which presentsgood erosion-resistant properties, e.g. any material as is known fromexisting erosion-resistant devices, e.g. helicopter tape, and/ormaterials such as W8607 from 3M, or 54994 PV3 from tesa SE. Preferably,said layers of erosion resistant material are approximately 0.1-0.5 mmdeep, preferably 0.3 mm deep.

The layer of bridging material 74 may be formed from an adhesive havinga relatively low bond strength between adjacent erosion resistant layers72. In this case, the erosion shield 70 may be formed by adheringsuccessive layers of erosion resistant material 72 together in a stack.Preferably, said at least one layer of bridging material isapproximately 0.05-0.25 mm deep, preferably 0.1 mm deep.

Preferably, the adhesive layer 76 arranged to attach the erosion shield70 to a blade leading edge 56 comprises a layer of acrylic-basedpressure sensitive adhesive, but it will be understood that any suitableadhesive layer may be used. The thickness of the adhesive layer may beapproximately 0.05-0.25 mm, preferably 0.1 mm.

In one embodiment, the bridging layer 74 may be provided as adouble-sided adhesive tape, wherein the bridging layer 74 is bonded tothe outer adjacent layer of erosion resistant material 72 using arelatively strong adhesive, and is bonded to the inner adjacent layer oferosion resistant material 72 using a relatively weak adhesive, suchthat in the event of delamination or peeling of the outer adjacenterosion resistant layer, the adhesive tape will be removed with theouter layer, thereby exposing a smooth surface of the underlying inneradjacent layer of erosion resistant material. In such a case, a pressuresensitive acrylic adhesive may be used on the side of the adhesive tapebonded to the outer adjacent layer, while a pressure sensitivesilicone-based adhesive may be used on the side of the adhesive tapebonded to the inner adjacent layer. Additionally or alternatively, theerosion shield 70 may be formed as a single formed or moulded structurehaving an interleaved arrangement of layers of materials havingdifferent structural properties, e.g. through co-extrusion of twothermoplastics having different material properties. The bridging layers74 may be formed of a material having a brittle or frangible structure,which can be relatively easily broken away from the erosion shield 70once an overlying erosion resistant layer 72 has been eroded.

The bridging layers 74 may comprise a thermoplastic material, and/or acoating material that will work as a semi-resistant erosion layer aswell as functioning as an adhesive between adjacent layers of theerosion resistant material 72.

Preferably, the bridging material 74 is be selected from any of thefollowing available materials: STEODUR-PUR-Kantenschutz from Bergolin;ALEXIT Leading Edge Protection 442-52 from Mankiewicz; XA258 fromHempel; or Oldodur Blade Finish from Relius. It will be understood thatany other suitable bridging material may be used in the erosion shield.

A further possible enhancement of the invention is illustrated in FIG.6. The erosion shield 70 is provided at the leading edge of a windturbine blade (not shown), the shield 70 extending along a portion ofthe length of the blade. The erosion shield 70 is divided into a seriesof sections 78 along the length of the shield 70, the boundary betweenadjacent sections 78 marked by a breakable, frangible or weakenedsection (indicated by dashed lines 80) of the shield 70 which extends ina direction transverse to the lengthwise direction of the erosion shield70. Such weakened sections 80 may comprise an area of the shield whichpurposively comprises layers 72,74 or reduced thickness, and/or aperforated or scored section of at least one of the layers 72,74 of theerosion shield.

The weakened sections 80 provide a pre-defined tear line for at leastone of the layers 72,74 of the erosion shield 70, such that anydelamination of the erosion shield layers 72,74 may be arranged to occurfor only at that section 78 of the erosion shield wherein a rupture orerosion occurs of the outermost layer of the erosion resistant material72. This provides for improved performance of the erosion shield 70, asthe entire outer layer of erosion resistant material 72 may be preventedfrom complete delamination as a result of only a single rupture point.

Accordingly, it will be understood that the outer leading edge surface70 a of the erosion shield may be formed by the exposed sections of aplurality of the layers of erosion resistant material 72, dependent onthe extent of the erosion which occurs in each of the predefinedsections 78.

Preferably, a greater number of sections are provided towards the end ofthe erosion shield provided at the tip end of the wind turbine blade.Due to the greater erosion experienced at the tip end of the blade asopposed to the root end of the blade, accordingly it is of greaterconcern that the portions of the erosion shield in this area are able toreact efficiently to any rupture or erosion of the outer layer of theerosion shield.

It will be understood that the individual sections of the erosion shieldwhen measured along the longitudinal direction of the erosion shield maydecrease in width moving from the root end of the erosion shield towardsthe tip end of the erosion shield.

Additionally or alternatively, the erosion shield may be constructed tohave a greater number of layers of erosion resistant material 72, andpossibly layers of bridging material 74, at the tip end of the erosionshield 70 than at the root end of the erosion shield 70. This ensuresthat there will be proportionally more erosion protection at the tip endof the blade, where the protection is most needed. Such a constructionprovides for a more effective erosion shield, and increases the timebetween maintenance operations to replace the erosion shield.

Additionally or alternatively, the erosion shield 70 may comprise afurther weakened tear line (not shown) provided along the leading edgeof the erosion shield 70, in a direction parallel to the lengthwisedirection of the shield 70. Such a longitudinal pre-defined tear linemay allow for the relatively easy delamination or peeling of theoutermost layer of erosion resistant material 72, as the oncoming windacts to force free ends of the outer layer adjacent the rupture pointaway from the leading edge.

It will be understood that such weakened sections are preferablyprovided within the layers of the bridging material 74, such that theerosion resistant properties of the layers of erosion resistant material72 are not affected by their presence. The weakened sections may beformed during the manufacturing process as an intentionally weakenedsection, e.g. a section of reduced thickness, or the weakened sectionsmay be formed through a post-production treatment operation, e.g. aperforation or scoring operation on a portion of the erosion shield.

A further alternative embodiment of an erosion shield according to theinvention is illustrated in FIG. 7. Similar to the embodiment of FIG. 4,the erosion shield 70 of FIG. 7 comprises an interleaved structure oflayers of erosion resistant material 72 and bridging material 74arranged for attachment to a leading edge of a wind turbine blade. Inthe embodiment of FIG. 7, a cavity or channel 82 is defined in thelayers of bridging material 74, said cavity or channel 82 located at thecentre point of layer of bridging material 74 along the leading edge ofthe shield 70, and extending in a direction parallel to the lengthwisedirection of the shield 70.

The channel 82 is preferably provided against the underside of theadjacent outer layer of erosion resistant material 72, and ensures thatthe outer layer of erosion resistant material 72 is not bonded to theunderlying layer of bridging material 74 along the leading edge of theerosion shield 70.

As a result, once the outer layer of erosion resistant material 72ruptures or erodes at the leading edge of the shield 70, the portions ofthe layer 72 not bonded to the underlying bridging material 74 willimmediately delaminate or peel away from the shield 70, presentingfluttering free ends of the erosion resistant material 72. This mayresult in a quicker delamination of the outer layer 72, as the oncomingwind is able to act upon a greater area of the underside of the outerlayer 72 to peel away the outer layer and the underlying bridging layer74.

The cavity or channel 82 may be a through going aperture extendingthrough each entire layer of bridging material 74, or may be a shallowindentation in the surface of the layer of bridging material 74 abuttingthe adjacent outer layer of erosion resistant material 72.

It will be understood that the width of the channel 82 may be of anysuitable dimension. While the embodiment of FIG. 7 shows the cavity 82as being defined to the leading edge, it will be understood that thecavity may extend through a greater portion of the bridging layers 74.

For example, with reference to FIG. 8, in one possible embodiment oferosion shield 70 according to the invention, successive layers oferosion resistant material 72 may be bonded to each other by an adhesivebridging layer 74 provided at opposite sides of the erosion shield 70.In such a case, the layer of bridging material 74 may comprise first andsecond adhesive tapes, indicated at 74 a,74 b arranged at the respectivesides of the erosion shield 70 between successive layers of erosionresistant material 72, namely at the upper suction side 84 and the lowerpressure side 86 of the erosion shield 70.

It will be understood that the cavity 82 and/or the weakened sectionwhich extends in a lengthwise direction along the erosion shield 70might not be provided along the leading edge of the erosion shield 70,for example it may extend along a line corresponding to a predicted areaof initial rupture of the outermost erosion resistant layer 72.

It will be further understood that the layers 72,74 of the erosionshield may be formed of several types of composite materials havingdifferent mechanical properties. For example, the erosion resistantmaterial 72 may comprise a resilient material composition at the leadingedge of the erosion shield 70, but may gradually be formed from acomposition having flexible mechanical properties adjacent to theleading edge, thereby being more flexible and easier to delaminate oncethe portion of the layer at the leading edge has been ruptured.

There is further provided a wind turbine blade 10 having an erosionshield 70 as described in any of the above embodiments, and a windturbine having at least one of said wind turbine blades 10. In onepreferred embodiment, the wind turbine blade 10 is arranged such that achannel (not shown) is defined in the body of the blade 10 along theleading edge 56 of the blade 10, the depth of the channel configuredsuch that the outer exposed surface 70 a of the erosion shield 70 isflush with the adjacent exposed surfaces of the wind turbine blade 10.Accordingly, the aerodynamic properties of the blade 10 are notsignificantly affected through use of such a layered erosion shield 70.

The erosion shield 70 herein described presents an improved erosionresistant system over the prior art. The dynamic delamination ofsuccessive layers of the erosion shield under action of the wind once anouter layer is eroded or ruptured provides an increased erosion shieldlifetime, and increased time between maintenance, repair and replacementoperations, with minimal impact on aerodynamic performance and noiseissues due to the partial erosion of the shield.

It will be understood that the figures shown are purely illustrative,and features shown, e.g. layer thicknesses, shapes, wind direction,etc., are not to scale.

The invention is not limited to the embodiment described herein, and maybe modified or adapted without departing from the scope of the presentinvention.

The invention claimed is:
 1. An erosion shield for a wind turbine blade, the erosion shield having an external surface exposed at a leading edge of the wind turbine blade and comprising: a plurality of layers of erosion resistant material, said plurality of layers arranged in a stack from an outermost layer substantially forming said external surface to an innermost layer arranged to be attached to a leading edge of a wind turbine blade, said plurality of layers bonded to adjacent layers in the stack, wherein said plurality of layers of erosion resistant material have a cohesive strength or tensile strength greater than an adhesive strength or bond strength between adjacent or subsequent layers, such that at least a section of an outermost exposed layer will delaminate or peel from said erosion shield under the action of wind when at least a portion of said section of said outermost exposed layer has been eroded or ruptured, to present a relatively smooth external surface of said erosion shield.
 2. The erosion shield of claim 1, wherein said plurality of layers are adhesively bonded to adjacent layers in the stack, and wherein said plurality of layers of erosion resistant material have a cohesive strength greater than the adhesive bond strength between adjacent layers.
 3. The erosion shield of claim 1, wherein said layers of erosion resistant material comprise an inner surface facing away from the external surface of the erosion shield and an outer surface facing towards the external surface of the erosion shield, wherein said erosion shield further comprises at least one layer of bridging material located between adjacent layers of erosion resistant material to bond adjacent inner and outer surfaces of adjacent layers of erosion resistant material in said stack.
 4. The erosion shield of claim 3, wherein said at least one bridging layer comprises a layer of adhesive.
 5. The erosion shield of claim 3, wherein said at least one bridging layer has a tensile cohesive strength less than a tensile cohesive strength of said layers of erosion resistant material.
 6. The erosion shield of claim 3, wherein said at least one layer of bridging material has a relatively stronger bond to an adjacent inner surface of a layer of erosion resistant material than to an adjacent outer surface of a layer of erosion resistant material.
 7. The erosion shield of claim 3, wherein a boundary between adjacent sections of said layers of erosion resistant material is defined by a perforated line or strip in at least one of said layers of erosion resistant material or in at least one of said layers of bridging material.
 8. The erosion shield of claim 7, wherein said perforated line or strip is in said layers of bridging material.
 9. The erosion shield of claim 3, wherein the erosion shield may comprise a perforated, weakened, scored or frangible section arranged in a lengthwise direction parallel to the leading edge of the erosion shield.
 10. The erosion shield of claim 3, wherein a cavity or channel is defined in said layers of bridging material, said channel or cavity arranged adjacent the underside of inner surface of the outer adjacent layer of erosion resistant material.
 11. The erosion shield of claim 10, wherein said channel or cavity is arranged at the leading edge of the erosion shield.
 12. The erosion shield of claim 3, wherein said at least one layer of bridging material is approximately 0.05-0.25 mm deep.
 13. The erosion shield of claim 12, wherein said at least one layer of bridging material is approximately 0.1 mm deep.
 14. The erosion shield of claim 1, wherein said layers of erosion resistant material are approximately 0.1-0.5 mm deep.
 15. The erosion shield of claim 14, wherein said layers of erosion resistant material are approximately 0.3 mm deep.
 16. The erosion shield of claim 1, wherein said erosion shield is arranged to extend in a lengthwise direction along a portion of a leading edge of a wind turbine blade, and wherein said layers of erosion resistant material are divided into a plurality of sections along the length of the erosion shield, wherein said sections of said erosion resistant layers are arranged to individually delaminate when at least a portion of an erosion resistant layer within a particular section has been eroded or ruptured.
 17. A wind turbine blade having an erosion shield as claimed in claim
 1. 18. A wind turbine blade as claimed in claim 17, wherein the wind turbine blade comprises a channel formed at the leading edge of the blade, said channel for receiving said erosion shield such that the erosion shield is provided flush with the surfaces of the blade adjacent the erosion shield.
 19. A wind turbine having at least one wind turbine blade as claimed in claim
 17. 